Pyrolytically decomposed resistor consisting of the elements carbon, oxygen and silicon



Ave. Resistance Jan. 22, 1963 Ave. Resistance Change (I) Change (7.)

.Ave. Resistance Change (7.)

K. GENTNER 3,074,817 PYROLYTICALLY DECOMPOSED RESISTOR CONSISTING OF THEELEMENTS CARBON, OXYGEN AND SILICON Filed April 26, 1957 I2 Fi I4 Fig. 3

I000 Hr. Load At 200C Ambient E Load Lite Test Results 16a 26a soc 460see see 160 e'oo e'oo lo'oo Time in Hours Hg- 4 Resistance change vs.

Ambient Temperature 2 For I000 Hrs. at Full Load o I I l I 0 so I00 lsozoo use Ambient Temperature (0) O g' 5 Overload Performance I MeqohmResistors with 5 Willis (2240-Volts) Applied 0 10'0 zoo zoo ebo eoo e'oe160 see she loco Time in Hours INVENTOR- KONRAD GENTNER ATTORNEY UnitedStates Patent ()fiice 3,074,817 Patented Jan. 22, 1963 3,074,817PYROLYTICALLY DECOMPOSED RESHSTOR CON- SISTIYG OE THE ELEMENTS CARBON,OXY- GEN AND SELICON Konrad Gentuer, Warminster Township, Bucks County,Pa., assignor to International Resistance Company, Philadelphia, Pa.

Filed Apr. 26, 1957, Ser. No. 655,383 2 Ciaims. (Cl. 117-216) Thisinvention relates to a pyrolytically deposited carbon film resistorwhich can be made to high resistance values and having improvedperformance characteristics.

Presently, carbon film resistors are formed by either pyrolyticallydepositing the carbon from a gas onto a ceramic base or by coating aninsulating base with a mixture of carbon particles in an insulatingbinder. These methods have been used for a long period of years, duringwhich time various methods and techniques have been developed to improvethe processes as Well as the final product, and the advantages of eachtype of resistor formed thereby are well known. However, there stillremains many inherent deficiencies in both types of resistors.

In the pyrolytically deposited carbon film, the resistivity of the filmis constant so that for a given area of film the only way of varying theresistance is by varying the thickness of the film. However, as the filmthickness is decreased to increase the resistance, the film becomes lessstable, i.e. subject to greater changes in value under operatingconditions. Thus, for a given area of film, the resistance valueobtainable in a stable film is limited. Furthermore, this type of filmis unstable at high temperatures. This is partially caused by the factthat at high temperatures the carbon oxidizes and goes oil as a gas.Although in the type of resistor having a resistance film composed of amixture of carbon particles in an insulating binder higher resistancevalues per unit area can be obtained than with a pyrolytically depositedcarbon film, this type of resistor is also unstable at hightemperatures. In fact, the normal operating characteristics of thisresistor are not as good as those of the pyrolytically deposited carbonresistor.

It is therefore an object of this invention to provide a stable highresistance value pyrolytically deposited carbon type film resistor andthe method of making the same. It is another object of this invention toprovide a pyrolytically deposited carbon type resistor which is stableat high temperatures. It is still another object of this invention toprovide a pyrolytically deposited carbon type resistor having improvedperformance characteristics. Other objects of the invention will in partbe obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, and thearticle possessing the features, properties and the relation ofelements, which are exemplified in the following detailed disclosure andthe scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings, in which:

FIGURE 1 is a cross-sectional view of the resistor of this invention;

FIGURE 2 is a cross-sectional view of a modification of the resistor;

FIGURE 3 graphically illustrates the load life characteristics of theresistor of this invention;

FIGURE 4 graphically illustrates the stability of the resistor withrespect to temperature and under a load; and

FIGURE 5 graphically illustrates the overload characteristics of theresistor.

In general, the invention resides in a resistance film composed of theelements carbon and silicon, individually and possibly in combination,and oxygen in the form of oxides of the other constituents. By varyingthe ratio of these elements in the film, the resistance value as well asother operating characteristics of the film can be varied. For example,by increasing the percentage of carbon in the film, the resistance valuewill be lowered. By decreasing the percentage of carbon or by increasingthe percentage of the oxides, the resistance of the film will beincreased. Therefore, the resistivity, resistance per unit area, is nolonger dependent on the thickness of the film but is varied by thecomposition of the film so that high resistance films can be obtainedwith the thicker and more stable films. By increasing the percentage ofthe oxides in the film, not only does the resistance of the filmincrease but a harder film is obtained which will withstand physicalabuse without affecting the resistance of the film. In addition, thefilm becomes much more stable with respect to high temperature, moistureand the electrical load applied to the film. By using multiple layers ofthis film, each having various ratios of the three elements, a resistorcan be provided having one layer which provides the desired resistancevalue and which is covered by another layer which provides a harddurable surface which is substantially unaffected by high temperatures,moisture and high electrical loading of the resistor.

Referring to FIGURE 1 of the drawing, the resistor comprises anon-conducting base it), commonly formed or" a ceramic of the typedescribed in United States patent to M. D. Rigterink, No. 2,386,633,issued October 9, 1945. A resistance layer 12 of the film composed ofthe elements carbon, silicon and oxygen is deposited on the surface ofbase 30 and contains a sufiicient amount of carbon, percentage-Wise, toprovide the desired resistance value. A second layer 14 or" the filmcomposed of the elements carbon, silicon and oxygen but having a muchhigher percentage content of oxygen than the resistance layer 12 isdeposited over resistance layer 12.. Although the resistor can beterminated by any of the well-known methods, the terminals are shown tobe in the form of metal caps 16 fitting over the ends of base 10 andcontacting second layer 14 with lead wires 13 extending from the capsin. This construction provides a resistor whose resistance value can bevaried by varying the percentage of carbon in the resistance layer 12 sothat high resistance values can be obtained with a relatively thick,stable film. In addition, the second layer 14 which has a high contentof oxygen provides the resistor with a hard durable film which is verystable with respect to high temperatures and the electrical load appliedto the resistor.

One method of testing the stability of a resistor with regard totemperature and the load applied to the resistor is known as the loadlife test and comprises heating the resistor to a known temperature andapplying the load to the resistor which is maintained over a long periodof time. The less the resistor changes in value during this test themore stable it is. FIGURE 3 shows the results of a load life test run ona group of resistors of this invention of various sizes and resistancevalues. The test was run on a group of resistors of this invention ofvarious sizes and resistance values. The test Was run at an ambienttemperature of 200 C. for a period of 1,000 hours with the resistors forcurves A and B being at their full rated load and the resistors forcurves C, D and E being at their full rated voltage. As can be seen fromFIGURE 3, the resistance change for these resistors was withinapproximately l% of their original value after 1,000 hours. Theseresults compare favorably with the results of load life tests made onpure deposited carbon resistors which are usually made at temperaturesof 40 C. or 70 C. When the pure deposited carbon resistor was tested ata vfull rated voltage.

higher temperature, 115 C., it was found that the resistor changed invalue approximately 10%. FIGURE 4 shows the eifect of temperature on theresistance value of the resistors of this invention when placed under aload. Curve A is for a group of the resistors of this invention whichwas placed under full rated load and curve B is for another group ofresistors which Was placed under It can be seen from these curves thatat the temperatures at which the pure deposited carbon resistor isnormally tested, 40 C. and 70 C., the change in resistance Wassubstantially negligible and even at the temperature of 250 C. thechange in resistance Was less than 3%. FIGURE 5 shows the efiect ofoverloading the resistor of this invention. For this test, a resistor ofa size which, for a pure deposited carbon resistor would be rated at 2.watts, was placed under a load of 5 watts. After 1,000 hours under thisoverload, it was found that the resistance value changed less than 1%.To test the physical durability of the film of this invention, the filmto there was no physical effect on the film.

When a pure deposited carbon film was tested in the same manner, thefilm completely disappeared in the matter of a few seconds. Thus thesetests shown that the film of this invention provides a resistor whichhas greater physical durability and is electrically more stable withrespect to temperature and the electrical load placed on the film thanother types of carbon film resistors. In addition, there is provided aresistor for use under high loads which is smaller in size than othertypes of carbon film resistors adapted to be used under such loads.

To form the resistor shown in FIGURE 1, the ceramic base 10 is placed ina sealed chamber having a gas inlet duct at one end and a gas outletduct at the opposite end. The chamber is then placed under a vacuum byexhausting the chamberthrough the outlet duct. A gas or mixture of gasescontaining the elements carbon, silicon and oxygen is admitted into thechamber to flow around the base. Prior to the admission of the gas, thechamber is heated to the decomposition temperature of the gas so that,as the gas passes around the base 10, it is decomposed to deposit a filmcontaining the elements carbon, silicon and oxygen on the surface of thebase. The gas first admitted to the chamber contains a high ratio ofcarbon so that the resistance film 12 is deposited on the base. When aresistance film 12 of the desired thickness has been deposited, theratio of the elements in the gas is changed in a manner as will beexplained later to provide a gas having a high percentage content ofoxygen. This gas is then decomposed to provide the outer layer 14.

The gas may be composed of a single compound containing the elementscarbon, silicon and oxygen or a mixture of two or three compounds andmay be obtained originally in a gaseous state or as the vapors of aliquid or solid. The carbon containing compound may be selected fromvarious hydrocarbons, both aliphatic and aromatic, alcohols, bothaliphatic and aromatic, aldehydes, ketones, organic acids, bothaliphatic and aromatic, ethers, esters and nitro, sulfo and halogenatedderivatives of these compounds. The silicon containing material may beselected from various halides, hydrides, alkyls and aryls of :silicon,halogenated hydrides, alkyls and aryls of silicon, :silicals, siloxanes,as well as -amino-, oxy-, or sulfoderivatives of these compounds. Theoxygen containing material may be selected from water, alcohols,aldehydes, ketones, organic acids and mixtures of these materials. Thefactors for choosing the particular compound, or compounds, to be usedare (l) the ease of obtaining the compound in its gaseous state, (2)that the compound can be decomposed at a reasonable temperature, and (3)that the material can be easily used without excessive danger.

Some examples of various gases which can be used are as follows:

(I) Three compound systems:

Silicon tetrachloride, Mesitylene and a mixture of water and methylalcohol. Heptane and a mixture of propyl alcohol and water. (11) Twocompound systems:

Trimethylchlorosilane and acetone. Silicon tetrachloride and a mixtureof propyl alcohol and water. (HI) Single compound system:Hexamethyldisiloxane. Methyltriethoxysilane.

The starting gas or gaseous mixture, should be selected to provide thedesired resistance layer 12. The particular starting gas may be changedto provide the outer layer 14 by adding or increasing the amount ofwater vapor or in the case of the two or three compound systems, bydecreasing the amount of the carbon containing compound.

The temperature to which the chamber is heated depends on thedecomposition temperature of the particular gas or gaseous mixture beingused and can be easily obtained by one skilled in the art from theliterature or experimentally. However, it has been found that the temperature will range between 1400 F. and 2000" F. for most gases,particularly those previously described. The chamber is maintained undera vacuum during the deposition process to remove undesirable reactionproducts so as to obtain an uncontaminated film as Well as to controlthe rate of flow of the gas into the chamber. Preferably, the vacuum isbetween .01 and 10 millimeters of mercury, depending on the desired rateof flow of the gas into the chamber.

FIGURE 2 shows a modification of the resistor. in which, in addition tothe resistance layer 12 and the outer layer 14, an under layer 20- isdeposited on the surface of the body 10 beneath the resistance layer 12.The under layer 20 is also composed of the elements carbon, silicon andoxygen and, like the outer layer 14, has a higher percentage content ofoxygen. It is well known in the art of pyrolytic deposition from a gasthat the material upon which the deposition is made may have a catalyticaffect on the rate of deposition and different materials affect thereaction differently. The ceramic materials usually used as the body 10for making resistors are composed of a mixture of materials which areexposed nonunifonmly along the surface of the body. Since certain ofthese materials have a greater aifect on the rate of deposition than theothers, the film deposited on the surface of the body is thicker at thepoints where these materials are exposed so that the film is not ofuniform thickness over the entire surface of the body. By firstdepositing the layer 20 on the surface of the base 10, there is provideda layer of more uniform composition on which the resistance layer 12 isdeposited. Thus the resistance layer 12 Will be of more uniformthickness to provide a resistance unit having improved operatingcharacteristics. This resistor is made in the same manner as previouslydescribed except that the gas first admitted to the chamber has a highpercentage content of oxygen. Once the under layer 14 is deposited, thegas is changed to contain the proper amount of carbon to provide theresistance layer 12 and then changed again to contain a high percentageof oxygen to deposit the outer layer 14.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are etficiently attained and,since certain changes may be made in carrying out the above method(process) and in the article set forth without departing from the scopeof the invention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing, shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. An electrical resistor formed by pyrolytically decomposing over anon-conductive base an atmosphere containing the elements carbon,silicon and oxygen, and depositing on the base a resistance filmconsisting essentially of the elements carbon, silicon and oxygen withthe content of carbon in said resistance film being suflicient toprovide a desired resistance value, and then pyrolytically decomposingover said resistance film a second at mosphere containing the elementscarbon, silicon and oxygen with the percentage content of the oxygen insaid second atmosphere being greater than that in said first atmosphere,and depositing on said resistance film a second film consistingessentially of the elements carbon, silicon and oxygen with thepercentage content of oxides in said second film being greater than thatin said first resistance film.

2. An electrical resistor formed in accordance with claim 1 in whichprior to pyrolytically decomposing the first atmosphere to deposit thefirst resistance film a third atmosphere containing the elements carbon,silicon and oxygen with the percentage content of oxygen in said thirdatmosphere being greater than that in the first atmosphere ispyrolytically decomposed over the base and a third film consistingessentially of the elements carbon, silicon and oxygen is deposited onsaid base with the percentage content of oxides in the third film beinggreater than that in the first resistance film.

References Qited in the file of this patent UNITED STATES P .TENTS1,365,331 McCulloch Ian. 11, 1921 2,028,776 Hibbert Ian. 28, 19362,105,166 Schwarzkopf Ian. 11, 1938 2,386,875 Morgan Oct. 16, 19452,442,976 Heany June 8, 1948 2,559,077 Johnson et al July 3, 19512,593,817 Waggoner Apr. 22, 1952 2,601,337 Smith-Johannsen July 24, 19522,664,364 Thom Dec. 29, 1953 2,683,673 Silversher July 13, 19542,697,025 Fulton et al. Dec. 14, 1954 2,698,257 Kronouer Dec. 28, 19542,726,172 Bennett et al. Dec. 6, 1955 2,762,717 Clark Sept. 11, 19562,771,565 Bryant et al. Nov. 20, 1956 2,778,743 Bowman Jan. 22, 19572,781,277 Dwyer Feb. 12, 1957 2,803,566 Smith-Johannsen Aug. 20, 19572,810,365 Keser Oct. 22, 1957 2,881,566 Badger Apr. 14, 1959' FOREIGNPATENTS 792,274 Great Britain Mar. 26, 1958

1. AN ELECTRICAL RESISTOR FORMED BY PYROLYTICALLY DECOMPOSING ANON-CONDUCTIVE BASE AN ATMOSPHERE CONTAINING THE ELEMENTS CARBON,SILICON AND OXYGEN, AND DEPOSITING ON THE BASE A RESISTANCE FILMCONSISTING ESSENTIALLY OF THE ELEMENTS CARBON, SILICON AND OXYGEN WITHTHE CONTENT OF CARBON IN SAID RESISTANCE FILM BEING SUFFICIENT TOPROVIDE A DESIRED RESISTANCE VALUE, AND THEN PYROLYTICALLY DECOMPOSINGOVER SAID RESISTANCE FILM A SECOND ATMOSPHERE CONTAINING THE ELEMENTSCARBON, SILICAN AND OXYGEN WITH THE PERCENTAGE CONTENT OF THE OXYGEN INSAID SECOND ATMOSPHERE BEING GREATER THAN THAT IN SAID FIRST ATMOSPHERE,AND DEPOSITING ON SAID RESISTANCE FILM A SECOND FILM CONSISTINGESSENTIALLY OF THE ELEMENTS CARBON, SILICON AND OXYGEN WITH THEPERCENTAGE CONTENT OF OXIDES IN SAID SECOND FILM BEING GREATER THAN THATIN SAID FIRST RESISTANCE FILM.